The Fv-4 gene of mice confers resistance to infection by MuLV. This dominant locus which maps to chromosome 12 has been found to be associated with DNA sequences that hybridize to an ecotropic MuLV env-specific probe. A physical map of the portion of the mouse genome that contains the Fv-4 related ecotropic env sequences has been determined. The relative locations of 12 restriction sites in a region of 23 kilobases of mouse genome that surround the Fv-4 MuLV env related sequence were obtained. The restriction map is unlike that of any known MuLV and thus the Fv-4 locus represents either a novel provirus or a fragment of a MuLV provirus containing only env related sequences. The physical map obtained will aid in the design of experiments to clone the segment of the mouse genome that contains the Fv-4 locus.